1. Field of the Invention
This invention relates to the manufacture of foamed seat cushions, in particular cushions made of polyurethane foam and more specifically is directed to a novel and useful method of breaking down some of the internal structure of the foam producing a softening of the foamed cushion.
2. Description of the Prior Art
It is known to manufacture foamed seat cushions in a female mold. A sheet of impervious fabric is introduced into the mold chamber to form the seat covering. A vacuum is applied to hold the seat cover in place against the mold wall. An in situ foaming process is then used to fill the mold chamber behind the seat covering with a foam which adheres to the back surface of the seat covering. A lid is applied to the mold while the foaming reaction takes place. A process of this type is disclosed in GB-PS 1,325,622.
After foaming has been completed, the finished cushion is extracted from the mold with the foam bonded directly to the inner surface of the seat covering. In order to give the foam the required resilience, the normal practice in manufacturing seat cushions is to crush the foam to break down some of the internal structure. This is conventionally done through a mechanical squashing or squeezing process. This mechanical squashing technique requires removal of the cushion from the mold and is potentially damaging to the fabric seat covering. Also, this mechanical squashing technique is incompatible with seat cushions having molded spring inserts.
Another technique is described in U.S. Pat. Nos. 4,579,700 and 4,717,518. This technique involves exposing the curing product to atmospheric pressure at a time when the cell walls have cured sufficiently to partially resist bursting by the pressure of expansion gas. This process requires careful timing of the exposure to atmospheric pressure. The curing foam undergoes a curing process wherein the cell walls are sufficiently strong to support the shape of the molded foam and avoid collapse of the molded foam when exposed to atmospheric pressure but are weak enough to burst by the internal pressure of hot expansion gasses within the cells and thus open the cells upon exposure to atmospheric pressure. This technique requires a special mold having an atmospheric vent valve and a means to open the valve at precisely the correct time.
Other techniques to soften foam include the shock treatment described in U.S. Pat. No. 3,239,585, and the secondary expansion process described in U.S. Pat. No. 4,360,484. Shock treatment requires immersing the foam in a nonsolvent aqueous liquid and impacting the liquid with a physical shock. Secondary expansion requires increasing the gas pressure within the cells and then heating the foam above its heat plastifying temperature to cause secondary expansion. Neither technique easily lends itself to the manufacture of cloth covered seat cushions.
All the methods described above either require post mold processing such as the physical squashing and shock treatment, or require complicated molding techniques such as atmospheric venting and secondary expansion. They do not permit the use of existing molds to perform an inmold crushing of the foam using conventional molding equipment.